1. Field of the Invention
This invention relates to a particle detection system and in particular to a detection system for distinguishing noncontaminant bubbles of a liquid from contaminant particles in the liquid.
2. Description of the Prior Art
During the processing of semiconductor wafers and the manufacture of integrated circuits, various liquids, such as HF, for example, are used for etching patterns in the wafers, and various solvents, such as isopropyl alcohol, TCE (trichloroethylene) and deionized water, are used to clean the wafers. During the etching or cleaning processes, contaminant particles in these fluids may be deposited on the wafers. The appearance of such particles significantly reduces the yield of the integrated circuits in production, particularly those circuit designs having very fine dimensions.
Various prior art devices have been employed to monitor particles in liquids. One prior art apparatus is shown in FIG. 1, wherein a liquid 10, which is drawn from a bath or pipe, is placed in a sample bottle or container 12. The bottle is then scanned with a laser beam 14 derived from a laser source 16, and the light is scattered from the particles on which it impinges in the liquid sample. The scattered light is detected by a detector 18. This arrangement is characterized by a number of problems. One problem is that the measurement is difficult and tedious to accomplish because it is not in situ. Also, since the sample is handled during the measurement process, additional contaminants may be introduced into the sample. Furthermore, the laser beam that passes through the bottle tends to become distorted thus leading to a loss of sensitivity of the detection system.
Another approach to detection of contaminant particles is shown in FIG. 2, wherein an airborne particle counter that allows measurement in situ employs a laser 20 for providing a laser beam 22 which is focused by means of a lens 24 through a window 26 of a small pipe section 28. The focused laser beam impinges on a sample volume 30 which is obtained from a fluid flowing through the pipe so that contaminant particles in the fluid can be detected. The beam and the light scattered by the particles passing through the beam exit from the pipe through a second window 32. As the scattered light is relatively very weak, the main beam which would normally saturate the detector is blocked by a stop element 34. The scattered light is focused through a lens 36 onto a detector 38. A problem which exists with this arrangement is that the focal spot of the laser is very small, generally less than a cubic millimeter, and therefore only a small fraction of the particles distributed in the liquid can be detected.
In another prior art arrangement, such as illustrated in FIGS. 3A and 3B a collimated laser beam 42 is derived from a laser source 40, which may be a helium-neon laser. The beam is passed through a cylinder lens 44 and through a window 48 associated with a container or cell 46 through which the sample liquid is passed. The window includes a beam truncating aperture 49 which narrows the beam to a desired diametric dimension. In the extinction mode, which is represented by FIG. 3A, the beam passes through an exit window 50 and the intensity of the beam is measured by means of a photodiode and preamplifier circuit 52. The extinction mode is used for the measurement of large contaminant particles. To measure the smaller particles, a scattering mode such as shown in FIG. 2 is employed where the main beam is blocked. For the measurement of smaller particles in the scattering mode, a collecting lens 54 is interposed between the exit window 50 and the photodiode and preamplifier circuit 52, as depicted in FIG. 3B.
In all of these prior art systems, there is an inability to distinguish the bubbles normally found in liquids from contaminant particles. With a small particle, the light of the beam is scattered because the index of refraction of the particle is different than that of the liquid medium. Similarly, the index of refraction of a bubble, which is about 1.0, differs from that of the surrounding fluid, which is typically 1.3 to 1.5. Consequently, a bubble will appear to the detectors of the prior art systems as an undesirable contaminant particle. However, a bubble is not deemed to be a contaminant, since it could pass through a filter which is used to stop contaminants. Therefore, to ascertain the accurate contamination level in a fluid, it is necessary to distinguish bubbles within a liquid from that of contaminant particles.